1. Field of the Invention
The invention relates to an air-fuel ratio control system and an air-fuel ratio control method for an internal combustion engine that control the air-fuel ratio of exhaust gas entering a catalyst.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2005-113729 (JP-A-2005-113729) recites an air-fuel ratio control system for an internal combustion engine. This air-fuel ratio control system has an upstream-side air-fuel ratio sensor provided upstream of a catalyst in the exhaust passage of the internal combustion engine and a downstream-side air-fuel ratio sensor (electromotive force type oxygen sensor) provided downstream of the catalyst. According to this air-fuel ratio control system, a feedback correction amount is calculated by performing a proportional integral derivative processing (so-called PID processing) to the deviation between the output value of the downstream-side air-fuel ratio sensor and the target value of the same output value (which corresponds to the target air-fuel ratio). This deviation will be referred to as “downstream-side deviation” where necessary. Then, the output value of the upstream-side air-fuel ratio sensor is corrected using the feedback correction amount calculated as above, and feedback control is performed on the amount of fuel injected from the injector using the corrected output value of the upstream-side air-fuel ratio sensor such that the air-fuel ratio equals the target air-fuel ratio.
In general, for example, a deviation unavoidably arises between the intake air flow rate detected by an airflow meter, which is used to determine the amount of fuel to be injected from the injector, and the actual intake airflow rate (the variation of detection by the airflow meter), and a deviation unavoidably arises between the required fuel injection amount that the injector is required to inject and the amount of fuel actually injected (the variation of injection from the injector). Such deviations will be collectively referred to as “error of fuel injection amount”. Further, the output value of a limiting-current type oxygen sensor that is typically used as the upstream-side air-fuel ratio sensor tends to include an error. Hereinafter, the error of fuel injection amount and the error of the upstream-side air-fuel ratio sensor will be collectively referred to as “error of intake and exhaust system” where necessary.
The aforementioned feedback control amount includes an integral term, that is, a value obtained by multiplying an integral value of the deviation, which is updated by integrating the downstream-side deviation, by a feedback gain. Therefore, even if the error of intake/exhaust system occurs, the error of intake/exhaust system may be compensated for dud to the integral term by performing the foregoing feedback control. As a result, the air-fuel ratio may converge and be made equal to the target air-fuel ratio. In other words, the value of the integral term (or the integral value of the deviation) may be used as a value representing the magnitude of the error of intake/exhaust system.
Such air-fuel ratio control systems perform an integral term learning process in which the value of the integral term (or the integral value of the deviation) as mentioned above is recorded while the recorded value of the integral term (hereinafter, this value will be referred to also as “learning value of the integral term”) is repeatedly updated (learned) at given time intervals.
Meanwhile, the value of the integral term (or the learning value of the integral term) converges to the value that accurately represents the magnitude of the error of intake and exhaust system (will be referred to as “target convergence value”). If the value of the integral term (or the learning value of the integral term) is equal to the target convergence value, it indicates that the actual air-fuel ratio which the air-fuel ratio control system treats as an air-fuel ratio equal to the target air-fuel ratio (will be referred to as “control center air-fuel ratio”) is actually equal to the target air-fuel ratio. When the control center air-fuel ratio is equal to the target air-fuel ratio, the error of intake and exhaust system may be properly compensated for, and thus the air-fuel ratio may be properly made equal to the target air-fuel ratio.
On the other hand, when the value of the integral term (or the learning value of the integral term) is deviating from the target convergence value, the control center air-fuel ratio becomes a value deviating from the target air-fuel ratio. In this case, there is a possibility that the error of intake and exhaust system may not be properly compensated for and thus the air-fuel ratio may not be properly made equal to the target air-fuel ratio. Therefore, when the control center air-fuel ratio is deviating from the target air-fuel ratio, it is necessary to make the value of the integral term (or the learning value of the integral term) converge to the target convergence value promptly.
According to the air-fuel ratio control system of JP-A-2005-113729, however, the value of the integral term is updated only by integrating the downstream-side deviation each time. Therefore, in particular, when the value of the integral term (or the learning value of the integral term) is largely deviating from the target convergence value, the value of the integral term (or the learning value of the integral term) does not converge to the target convergence value promptly.